Gas laser including cathode element

ABSTRACT

A gas laser including a cathode member comprising a platinum containing base member having an emissive coating deposited thereon comprising at least 90 percent barium zirconate, remainder barium oxide.

Patented April 4, 1972 3,653,960

2 Sheets-Sheet 1 INVENTOR 0. MAC NAIR United States Patent MacNair 11451Apr. 4, 1972 [54] GAS LASER INCLUDING CATHODE [56] References CitedELEMENT UNITED STATES PATENTS [72] Invent Mama" Berke! Helghts 3,155,86411/1964 Coppola ..117/223 x [73] Assignee: Bell Telephone Laboratories,Incorporated, 1,823,984 9/1931 Nocolson... ..117/219 Murray Hill,Berkeley Heights, NJ. 3,188,236 6/1965 Speros ..117/220 [22] Filed: Mar.23, 1970 1 Primary Examiner-Ralph S. Kendall [21] P 21,854 Attorney-R.J. Guenther and Edwin 1B. Cave Related U.S. Application Data [63]Continuation of Set. NO. 847,516, July 18, 1969, i [57] ABSTRACTabandoned, which is a continuation of Ser. No. A gas laser including acathode member comprising a 568,497, July 28, 1966, abandoned. platinumcontaining base member having an. emissive coating deposited thereoncomprising at least 90 percent barium zir- [52] U.S. Cl ..117/221, 117/223, 252/520, conate, remainder barium oxide.

252/521, 313/346, 330/41 [51] Int. Cl ..1-101s 3/22 Qrayylnsjigu ec 1 1wsman-6,221 .1422521252152 1 31 GAS LASER INCLUDING CATIIODE ELEMENTThis is a continuationof application, Ser. No. 847,516, filed July 18,1969, now abandoned which was a continuation of copending application,Ser. No. 568,497, filed July 28, 1966 of operating in oxygen containingambients.

Recently, considerable interest has been generated in the electronicsindustry in laser devices which operate in a reactive gaseous ambient.In the operation of such devices, it is conventional to employathermionic tube including a cathode element capable of performing athigh current levels for the purpose of ionizing the gaseous medium.Unfortunately, cathodes presently marketed commercially are onlyoperative at the desired current levels in the absence of reactive electronegative gases such as carbon dioxide, oxygen, water vapor, etc.

In accordance with the present invention, a cathode element capable offunctioning at high current levels either in vacuum or in an oxidizingambient is described. This element, which includes a coating of at least90 percent, by weight, of barium zirconate upon a noble metal base oralloy thereof exhibits a degree of current density for a givenconfiguration and operating conditions which is comparable to thatattained by any conventional cathode structure capable of functioningonly in vacuum. Operation of the described cathode in vacuum issuccessfully effected by including up to percent, by weight, of a bariumcompound in the coating, such compound decomposing upon a vacuum stationto yield barium oxide.

The invention will be more readily understood by reference to thefollowing detailed description taken in conjunction with theaccompanying drawing wherein:

FIG. 1 is a front elevational view in section of a cathode structureutilizing a cathode element of the invention;

FIG. 2 is a perspective view of the cathode structure described herein;and

FIG. 3 is a schematic representation of a typical gas laser systemutilizing the inventive cathode structure.

With reference now more particularly to FIG. 1, there is shown astructure containing the inventive cathode element. Such structureincludes outer envelope 11, containing cylindrical heat shield 12comprising platinum, rhodium, iridium or alloys thereof having containedtherein cathode element 13. Element 13 is hairpin shaped and comprises abase of platinum, rhodium, iridium or alloys thereof having an emissivesurface comprising at least 90 percent, by weight, barium zirconate of athickness ranging from 0.5 mils. Paired electrical leads 14 and 15 makeconnection with cathode 13 and pass through glass base 16 which ishermetically sealed with envelope 11. Similarly, paired electrical leads17 and 18 make connection with heat shield 12 and pass through base 16.

The structure and cathode element described herein may be more fullyappreciated by reference to FIG. 2 wherein there is shown a perspectiveview of the element. Shown in the figure is base member 21 havingdeposited thereon emissive coating 22 covering the entire surfacethereof.

FIG. 3 is a schematic representation of a carbon dioxidenitrogen laserutilizing the described cathode. The figure shows a cathode structure31, of the type shown in FIG. 1, including cathode element 32 and heatshield 33, and an anode structure 34. Cathode 31 and anode 34 are bothshown connected hermetically to glass tube 35 having a pair of opticallypolished Brewster angle windows 36 and 37. Cathode structure 31 is alsoconnected to a vacuum pump and gas source (not shown) by means ofconduit 38 and to a heat source 39 through paired electrical leads 40and 41, respectively, which are connected to element 32. Heat shield 33is connected by means of paired electrical leads 42 and 43 to thenegative pole of a d-c source 44. Anode 34 is connected by means ofpaired electrical leads 45 and 46 to the positive pole 47 of the said dcsource.

In the operation of the device, cathode structure 31 is evacuated to apressure of order of 10" millimeters of mercury. Following cathodeelement '32 is heated to a temperature of approximately 900 C., amixture of carbon dioxide and nitrogen (0.5 torr. of each) admitted tothe system via conduit 38 and a difference of potential impressedbetween anode and cathode, so resultingin ionization of the gaseousmedium. The radiation developed within the discharge plasma glass 33then passes through windows 36 and37.

A general outline of a method suitable for use in the manufacture of l acathode element of this invention is set forth below. Certain operatingparameters and types of starting materials are indicated.

The base material selected for the practice of the invention may beselected from among platinum, iridium, rhodium or alloys thereof. Thematerial employed is obtained from commercial sources in wire, screenorany other solid form suitable for the desired configuration andoperating characteristics. The grade of metal or alloy chosen should beas nearly pure as practicable so as not tocontain any contaminants whichmay impair the emitting characteristics of the final structure. Theemitting mixture selected for use in the novel structure includes atleast percent, by weight, barium zirconate. This material may beobtained from commercial sources and is typically of reagentgradepurity.

Although the particle size of the emitting mixture is not critical,ageneral preference exists for fine particles generally ranging from 25to microns.

In those instances when it is desired to operate the described cathodestructure not only in an oxidizing ambient, but also in vacuum, it isnecessary to add up to 10 percent, by weight, of the emitting mixture ofa well-known emitting composition. These materials usually contain abarium compound which will break down on a vacuum station to yieldbarium oxide. For the purpose of the invention, any barium compoundwhich will thermally decompose at a temperature of less than 1,000 C.(and which will not react with barium zirconate) to yield barium oxideis suitable. Such materials include the single carbonate material,barium carbonate, the double carbonate material, coprecipitated bariumstrontium carbonate; and the triple carbonatematerial coprecipitatedbarium strontium calcium carbonate. In general, it has been found thatthe double carbonate is to be preferred over the single and that littlefurther advantage is gained by the use of the triple carbonate. Thedouble carbonate most commonly available for this purpose is acoprecipitant of equimolar portions of barium carbonate and strontiumcarbonate. The particle size of this emitting mixture is notcritical, apreference again existing for fine particles. A commercially availablecoprecipitant containing particles, 90 percent of which are smaller than10 microns has been found satisfactory.

In addition to the barium zirconate with or without the carbonate, abinder material may be added. The material selected may benitrocellulose or any suitable material well known to those skilled inthe cathode art such as isobutyl methacrylate, acetone solutions ofstearic acid, etc. Binders are added to the mixture in minimumquantities (2 to 4 percent, by weight, of the total mixture) to assuremaximum density.

Thefollowing is an outline of the procedure to be followed in producinga cathode element from the above materials.

The base material isinitially cleaned by conventional vapor degreasingtechniques well known to those skilled in the art. The cleaned base maythen be stored until required.

The emitting mixture is produced by mixing barium zirconate with abinder alone or in combination with a portion of single, double ortriple carbonate in a ball mill for time periods ranging from 50 to 100hours. The nextstage in the fabrication of the inventive structureinvolves coating the base with the emitting mixture by conventionalspraying or dipping techniques. After deposition of the emissive coatingmaterial. the cathode is mounted as shown in FIG. 1 and the structureheat treated in vacuum.

In brief, the procedure employed consists of sealing the element in avacuum station and evacuating to a pressure ranging from 10 to 10millimeters of mercury. The cathode is then heated to about l,00O C. forthe purpose of eliminating occluded gases, and in those cases where acarbonate is present in the emitting mixturefor the additional purposeof convening the carbonate to the corresponding oxide. For operation invacuum, the structure is now completed and by heating to a temperaturewithin the range of 800-900 C. and applying an anode potential currentmay be drawn. For operation in an oxidizing ambient, the structure isfilled with the desired gaseous medium, for example, air, carbondioxide, oxygen, water vapor, carbon dioxide-nitrogen mixtures, etc. Thepartial pressure of the gaseous medium in the structure may range from0.0l torr. to atmospheric pressure. Current may then be drawn in themanner described.

In order that those skilled in the art may more fully understand theinventive concept herein present, the following examples are given byway of illustration and not limitation.

EXAMPLE I A cathode element of the type shown in FIG. 1 was prepared asfollows: A platinum-rhodium screen (45 X 45 mesh containing 0.008 inchesdiameter platinum wire) 4 X inches of 99.9 percent purity, obtained fromcommercial sources, was vapor degreased and cleaned ultrasonically byconventional techniques with trichloroethylene. Next, the screen wasrinsed in acetone. An emitting mixture was prepared by mixing 50.0 gramsof reagent grade barium zirconate with 10 cubic centimeters ofnitrocellulose solution and 10 cubic centimeters of amyl acetate for 50hours in a conventional ball mill. The resultant mixture containingparticles ranging in size from 25 to 100 microns was next sprayed byconventional techniques upon both surfaces of the platinum screen untila coating thickness of 1 mil was obtained. Next the platinum screen wasmounted in a glass envelope by spot welding techniques and the resultantstructure evacuated to a pressure of 10 millimeters. Then, the cathodeelement was heated to l,000 C. to remove occluded gases, cooled to roomtemperature and filled with a carbon dioxidenitrogen mixture to apressure of 1 torr. (one-half torr. of each). Finally, the structure washeated to 1,000" C an anode potential applied and current drawn. Thestructure drew 0.05 ampere/cm. for greater than 1,000 hours.

EXAMPLE I] The procedure of Example I was repeated with the exceptionthat 45 grams of barium zirconate and 5 grams of coprecipitated barium,strontium carbonate were employed. At 1,000 C. the resultant structureevidenced a current carrying capacity of 0.05 ampere/cm. in vacuum, uponapplication of an anode potential.

While the invention has been described in detail in the foregoingspecification and drawing, the aforesaid is by way of illustration onlyand it is not restrictive in character. The several modifications whichwill readily suggest themselves to persons skilled in the art are allconsidered within the broad scope of the invention, reference being hadto the appended claims.

I claim:

1. Gas laser including a cathode member, an anode member, means forintroducing a reactive electronegative gas thereto, means for heatingsaid cathode member and means for impressing a difference of potentialbetween said cathode and anode members, characterized in that saidcathode member comprises a base member selected from the groupconsisting of platinum, rhodium, iridium and alloys thereof,

said base member having an emissive coating consisting essentially of atleast percent, by weight, barium zirconate, remainder alkaline earthoxide.

2. Device in accordance with claim 1 wherein said emissive coatingcomprises percent, by weight, barium zirconate.

2. Device in accordance with claim 1 wherein said emissive coatingcomprises 100 percent, by weight, barium zirconate.